The present invention relates to respective improvements of an ultrasonic motor that comprises a vibrating body periodically vibrating due to the expansion and contraction vibration of a piezoelectric element to thereby form a standing wave thereon and a power transmission projection provided at an intermediate position between the loop and node of the standing wave of the vibrating body, and to an electronic apparatus and an analog type timepiece each of which uses the ultrasonic motor.
Recently, in the field of a micromotor, attention has been drawn toward an ultrasonic motor which uses a piezoelectric element as a drive source therefor. As the ultrasonic motor there are known a type wherein different resonant modes of a rectangular flat plate-like vibrating body are combined with each other, a type wherein a travelling wave is caused to occur on an annular vibrating body by electric excitations whose phases differ 90.degree. from each other, etc.
Also, from the viewpoint of causing the production of a drive force in the normal and reverse directions by the use of a single power source, there is known a type causing the occurrence of standing waves on a vibrating body (see Japanese Patent Application Laid-Open Publications Nos. Hei 2-287281 and Hei 8-107686).
FIGS. 9A and 9B each illustrate a main part of the standing wave type ultrasonic motor, FIG. 9A illustrating a developed structure of the circumferential surface and FIG. 9B thereof illustrating a planar structure.
A main part of the ultrasonic motor is comprised of a piezoelectric element 11 having divided portions prepared by circumferentially dividing the element 11 into twelve equal sectorial portions, wherein each pair of adjacent divided portions are processed so that each pair may be alternately inverted in the polarization direction, a vibrating body 12 joined to the piezoelectric element 11, power transmission projections 13 each provided on a circumferential surface of the vibrating body 12 at a position corresponding to the boundary between the paired divided portions, and a moving body 24 abutted on the power transmission projections 13.
In order to move the moving body 24, as illustrated in FIG. 9B, a voltage is applied to a group of the divided portions (11a or 11b) of the piezoelectric element 11 which have alternately been disposed to thereby cause the occurrence of standing waves on the vibrating body 12. Here, the power transmission projection 13 is located at an intermediate position between the loop and node of the standing wave occurring on the vibrating body 12 every half-wavelength.
At this time, when each power transmission projection 13 is located on the crest side of the standing wave, a forward end thereof abuts on the moving body 24 to thereby apply a frictional force thereto, and the power transmission projection 13 located at an intermediate position between the trough and node of the standing wave is kept out of contact with the moving body 24. Therefore, the moving body 24 is rotated in one direction.
However, when miniaturizing the ultrasonic motor, the amplitude of the excited standing waves is very small, and respective sliding surfaces of the vibrating body 12 and moving body 24 each have convexities and concavities due to the microscopic undulation and roughness.
FIG. 10 illustrates the surface configurations of the vibrating body and moving body and contacted states of the projections with the surface of the moving body.
Namely, when convexities and concavities exist on the sliding surfaces of the vibrating body 12 and moving body 24, not only the power transmission projections 13a on the crest side of the standing waves but also a part of the ordinarily non-contacted power transmission projections 13b on the trough side of the standing waves is brought into contact with the moving body 24 to thereby apply a frictional force in a direction opposite to the advancing direction of the moving body 24. This becomes the loss of the energy at the time of converting the vibration energy of the vibrating body 12 to the rotating force of the moving body 24, raising the technical problem of impairing the rotation performance of the ultrasonic motor.
Also, while the operation of cutting the surface of the disk-shaped vibrating body 12 into projecting configurations is necessary for producing the power transmission projection 13, since a complex grinding procedure and a significantly large amount of time are necessary for producing a large number of projecting configurations, there exists the problem that the manufacturing process becomes complex.